1. Field of Invention
The invention relates in general to a switching audio power amplifier and a method for pop noise suppression, and more particularly to a switching audio power amplifier for suppressing pop noise by gradually turning on or off a plurality of control switches.
2. Related Art
When an audio power amplifier is powered on or off, an uncomfortable pop noise may be induced if the switching is not smooth enough during the period when an audio signal suddenly appears or disappears. FIGS. 1A to 1D show waveforms generated after a pulse or pulses pass through a low-pass filter when the pulse or pulses suddenly appear or disappear. As shown in FIG. 1A, when there is only one pulse in an input signal, two inrushes (up and down inrushes) may appear after the input signal passes through the low-pass filter. As shown in FIG. 1B, when there are a series of pulses, no inrush is generated when the input signal passes through the low-pass filter. However, there must be a start (power on) and an end (power off) in the series of pulses in FIG. 1B, and the corresponding start and end signals are shown in FIGS. 1C and 1D. So, as shown in FIGS. 1C and 1D, the inrushes will still appear after the input signal passes through the low-pass filter. If the inrushes reach the levels that may be heard by human ears, an annoying pop noise is generated.
Typical audio power amplifiers are classified into analog power amplifiers with changeable amplitude of the output signal, and switching audio power amplifiers with fixed amplitude of the output signal. The methods of pop noise suppression in the analog power amplifier have been disclosed in, for example, U.S. Pat. Nos. 4,788,508, 5,682,121; 6,157,726 and 6,516,067. However, these patents provide the circuits working in conjunction with analog power outputs, and are thus not suitable for switching audio power amplifiers that are originally discontinuous in nature.